It is known that a radio frequency test instrument, for instance a spectrum analyzer, shows hardware imperfection such as a frequency response being not perfectly flat.
The frequency response can be measured at the customer in the field using a so called “self-alignment” function or in the factory using a so called “factory calibration”. Once the frequency response has been measured, it can be compensated during operation of the radio frequency test instrument by adjusting at least one filter appropriately which inverts the frequency response. For instance, an equalization filter is applied for compensating purposes. The goal is to have a very fine resolution for the equalization filters over the measurement bandwidth and a very good fitting equalization filter for the radio frequencies to be processed by the radio frequency test instrument.
The frequency response is influenced by an intermediate frequency part and a radio frequency part. Usually, the part of the frequency response originating from the radio frequency path of the analyzer is relatively flat within a bandwidth of 1 GHz, for instance. On the other hand this flat frequency response is superposed by a frequency response of the intermediate frequency with higher ripples over the corresponding bandwidth. However, both influences cannot be measured separately as they depend on each other. In addition, the resulting frequency response of the radio frequency test instrument can be separated in a so called fast influence and a so called slow influence for every single radio frequency tested.
In general, different methods are known to measure the amplitude of the frequency response to be used for calibrating the radio frequency test instrument.
For instance, a signal generator and a power meter are used. The signal generator transmits a single tone at a known frequency. The actual power of the tone is measured with the power meter in order to determine the frequency response. However, this measurement takes a long time for determining the frequency response for a high bandwidth.
Another method relates to the usage of an external comb generator that has to be calibrated initially for the different radio frequencies. However, the calibration takes some time and, further, a void calibration of said comb generator results in radio frequency test instruments being calibrated inappropriately. Particularly, factory calibrated radio frequency test instruments would have to be returned for recalibration purposes. In addition, a drifting of the comb generator over the time has to be taken into account while calibrating radio frequency test instruments.
Another method relates to an intermediate filter calculation as a fine frequency response measurement is performed such that the frequency response can be calculated with high accuracy for a few radio frequencies in order to set the equalization filter with regard to the determined frequency response. Then, a coarse frequency response measurement is performed while the equalization filters are set and applied. The frequency response is calculated again and the settings for the equalization filters are determined again. Afterwards, both settings for the equalization filters are combined and applied. However, the calibration time increases with the number of fine frequency response measurements performed, in particular the number of different radio frequencies used for the fine frequency response measurement. In addition, the fine and coarse measurement depend from each other resulting in a possible propagation of errors. Moreover, the equalization filters have to be set and applied for the coarse measurements which may also result in an additional error source.
Accordingly, there is a need for a method for calibrating a radio frequency test instrument that can be done fast, in a less complex manner and with a high accuracy.